Interactions between the pulvinar and cortex are functionally and numerically important. Yet, the organization of this system at the level of microcircuitry is poorly understood by comparison with other major cortically-linked centers, such as the basal ganglia and the specific sensory thalamic nuclei. In this project, we will use high resolution serial axon reconstruction and neuron analysis to test and refine the general hypothesis that principles derived from specific sensory pathways do not apply to associational thalamic nuclei and to their cortical connectivity. Preliminary results illustrate the promise of this approach. For example, we now have evidence for two types of corticopulvinar terminations: 1) small, spherical arbors with large beaded endings, and 2) elongated terminations, diverging over several millimeters, with spinous terminal specializations (see Appendix:Rockland, 1994a). These may originate from two previously identified populations of corticopulvinar neurons; namely, giant and medium pyramids of layer 5. We hypothesize that events sampled by pyramids with wide dendritic fields are "compressed" to a delimited terminal focus in the pulvinar, whereas more localized analysis (performed by the more numerous medium pyramids) is distributed over an assembly of pulvinar neurons. Different synaptic mechanisms are also likely to be associated with the large beaded vs. spinous specializations. Four experiments are proposed. Two will use PHA-L to identify corticopulvinar and pulvinocortical axons. These will be analyzed as projection foci, and reconstructed in serial sections to determine such parameters as axon caliber, branching, arbor size and shape, and density of terminations. A third experiment will use combined in vivo/in vitro filling to carry out a detailed mapping of the two populations of corticopulvinar projection neurons, in terms of morphometric and geometric features of their dendritic phenotype. We will apply quantitative methods to determine the density of giant corticopulvinar neurons, their proportion to medium corticopulvinar neurons, and area-specific differences. A fourth experiment addresses the topographic organization and convergence of corticopulvinar connections. Two anterograde tracers will be injected into visuotopically corresponding foci of two cortical areas. These data may illuminate questions arising from psychophysical, physiological and clinical observations, as these relate to the role of the pulvinar in selective visual attention. The identification of two pyramidal phenotypes, distinguished by soma size, dendritic arbor, and extrinsic axon terminations, may be relevant 1) to separate subcomponents of attention mechanisms; and 2) as a connectionally-specific model supporting further molecular and functional characterization. In other systems such as the geniculocortical, axon microcircuitry has provided a useful baseline for design in interpretation of studies related to plasticity effects, and the proposed program of research may promote similar applications.